An inverter is employed to invert direct current power into alternating current power. In one form of such an inverter, a plurality of inverter switches are connected to a DC link and are controlled by pulse-width modulated (PWM) pulses. The inverter switches produce output PWM pulses which are supplied to a filter. The filter reduces or removes harmonics of an inverter fundamental frequency so that an AC output approximating an AC sinusoidal waveform is produced.
A common prior art PWM inverter control includes a comparator which compares a triangular waveform to a reference sinusoidal waveform. The reference sinusoidal waveform has the desired shape of the AC output from the inverter. The output of the comparator comprises a PWM waveform having a substantial frequency component at the desired inverter fundamental frequency.
Filters, particularly those which are arranged to filter out the lower frequency harmonics such as the fifth and seventh harmonics, are expensive, heavy and large. Many inverter systems control the inverter switches so as to reduce or eliminate a preselected number of the lower frequency harmonics in the inverter AC output. Thus, the filter is instead arranged to filter out higher frequency harmonics so that the expense, weight and size of the filter are accordingly reduced.
One system for supplying PWM pulses to the switches of an inverter is shown in U.S. Pat. No. 4,527,226 to Glennon In this system, a memory stores a plurality of PWM pulse patterns (or "angle sets") each having a unique set of switching angles. The switching angles of the stored angle sets are predetermined analytically so that selected harmonics in the inverter output can be minimized. A set of switching angles is selected from the memory based in part upon the power factor at the inverter output. This selected set of switching angles is supplied to the inverter switches so that the harmonic content in the output of the inverter is reduced.
Inverter systems utilizing stored patterns have limited harmonic reduction capability because only a limited number of angle sets are available for use. In such systems, if a power factor is sensed for which there is no stored angle set, the stored angle set associated with the power factor closest to the sensed power factor is selected. However, use of this angle set does not result in minimum harmonic content. Also, many prior art inverter systems assume that the DC voltage on the DC link is stiff, i.e., has no ripple. However, an inverter is bidirectional in the sense that any harmonic ripple in the output of the inverter passes back through the inverter to the DC link. This ripple on the DC link modulates the output of the inverter to produce further harmonic content in the inverter output. Inverter systems utilizing stored angled sets are typically not arranged to compensate for this additional harmonic content. Furthermore, many stored angle set systems do not directly measure harmonic content in the inverter output, but develop an approximation thereof, and hence cannot completely eliminate controlled harmonics.
A system in which the DC link is not assumed to be stiff and which senses the harmonic content in the output of an inverter is disclosed in Kirchberg, et al., U.S. Pat. No. 4,994,956. In this system, parameters of the inverter output are used to select an initial angle set from a memory storing a plurality of such angle sets. The Fourier coefficients of selected harmonics in the inverter output are determined and these Fourier coefficients are used to adjust the switching angles of the selected angle set in order to reduce the harmonic content in the inverter output regardless of whether this harmonic content arises from the PWM switching of the inverter switches or from harmonics passed back through the inverter to the DC link. This system has the further advantage in that harmonic reduction is not limited by the number of stored angle sets.